The worst movie, hands down, that I've ever seen:
Adaptation (starring Nicolas Cage, Nicolas Cage, and Meryl Streep).
There are different kinds of bad movies. Some that come to mind:
Airplane 2 (worst sequel; so bad I feel fairly certain that they were intentionally trolling those who paid to go see it)
Road Trip (worst rip-off of another movie that was actually good and immediately preceeded it, that being Overnight Delivery)
How to Lose a Guy in 10 Days (worst rom-com; was neither romantic or comedic, it was just vicious)
Eternal Sunshine of the Spotless Mind (biggest WTF; a couple hours of my life that I'll never get back)
Breakfast at Tiffany's (very famous, starring one if the greatest actresses of all time, but just was not good, and featured one of the most racist character depictions in Hollywood history by Mickey Rooney)
Terminator (worst and most pointless movie that had an incredibly good sequel)
Riding in Cars With Boys (most depressing movie, was not advertised as a such, no real redeeming quality, thanks Drew Barrymore)
Hot Tub Time Machine (juvenile and vulgar without being funny)
Of course, I've seen lots of other bad movies over the years, but these ones were remarkable for being memorable, which most of the others were not.